The use of crosslinking agents in the synthesis of polymers is known in the art. For example, compounds such as glycerol, sorbitol, 3,5-dihydroxy-methylbenzyl alcohol and pentaerythritol find wide applicability as polyfunctional crosslinking agents.
A crosslinking agent which is commonly used is 2-ethyl-2-hydroxymethyl-1,3-propanediol or trimethylolpropanetriol. This crosslinking agent, or trifunctional crosslinking agents as it may be more accurately described is used in particular to produce crosslinked polyesters.
It has additionally been known to functionalize trimethylolpropanetriol further to produce other trifunctional crosslinking agents. For example, it is known to treat 2-ethyl-2-hydroxymethyl-1,3-propanediol with acryloyl or methacryloyl chloride to produce 2-ethyl-2-hydroxymethyl-1,3-propanediol triacrylate and 2-ethyl-2-hydroxymethyl-1,3-propanediol trimethacrylate. The resulting polyfunctional crosslinking agents find particular applicability in crosslinking reactions with acrylates and methacrylates.
In such methods, the trimethylolpropane triacrylate or TMPTA and trimethylolpropane trimethacrylate or TMPTMA are then reacted with acrylate and methacrylate monomers, preferably in the presence of a sufficient amount of free radical initiator to facilitate polymerization. Such methods are often used to produce crosslinked thermoset polymers. The resultant thermoset polymers find applicability, e.g., in making automobile coatings and composites. Additionally, crosslinked thermoset polymers are used in bulk or thermoset monomers may be diluted with other monomers to produce adhesives or composite materials.
Although both TMPTA and TMPTMA are conventionally utilized as crosslinking agents with both acrylate and methacrylate monomers; there are inherent disadvantages associated with the use thereof. In particular, due to the size of TMPTA or TMPTMA and the symmetry of these molecules, only a small separation exists between the reactive ends of the acryloyl or methacryloyl moieties. Because of this close proximity, these reactive termini may react with themselves in addition to reacting with other acrylate or methacrylate monomers in the polymerizing melieu. The former produces cyclic structures (10 membered rings) and is undesirable since it wastes the double bonds of the crosslinking agent. The latter reduces the overall efficiency of the crosslinking agent. Additionally, internal cyclization often results in a weakened polymeric network which is undersirable if the crosslinked polymer is to be industrially useful.
In order to minimize the recognized problems associated with such crosslinking agents, it had been contemplated in the art to increase the overall concentration of the crosslinking agent in the polymerization mixture as a means of enhancing the degree of crosslinking. However, even at high crosslinking agent concentrations, the resultant polymers may be brittle and mechanically weak. Additionally, the increased concentration of the crosslinking agent renders the polymers undesirably expensive to produce. For instance, it is described, e.g., in Matsumoto et al, "Gelation in the copolymerization of methyl methacrylate with trimethylolpropane trimethacrylate."; Eur. Polym. J., Vol. 25 (4), pp. 385-389 (1989), that the crosslinking efficiency of trimethylolpropane trimethacrylate is only 18% and that 82% of the polymer mixture had internally cyclized despite the fact that an excess of methyl methacrylate was present in the polymerization mixture.
Furthermore, K. Dusek in Developments in Polymerization-3, R. N. Howard (editor), Applied Science Publishers, Ltd., London, p. 63, (1982), describes that when methylene bis-acrylamide is used to crosslink polyacrylamide hydrogels at a 10-20% acrylamide crosslinker concentration crosslinking efficiencies are only 10-20%. Such low crosslinking efficiencies may result in polymers which are not structurally load bearing.
Thus, given the described state of the prior art, it is clear that improved methods for producing crosslinked polymers, and improved crosslinked polymers resulting from these methods would be highly desirable.